SOx , NOx , and particulate removal system

ABSTRACT

A method and apparatus for controlling emissions of a fossil fuel fired boiler including a high temperature fabric filter house with an SCR catalyst situated therein for receiving flue gases along with an injected ammoniacal compound and sorbent. The sorbent reacts with the SO x  while the ammoniacal compound reduces the NO x  in the presence of the SCR catalyst inside the high temperature fabric filter house. Both the SO x  and the particulates are removed upstream of the SCR catalyst to diminish the problems of SO 2  or SO 3  poisoning of the catalyst and erosion and fouling of the catalyst with the fly ash. Since the sulfur oxides and particulates are removed prior to the heat recovery system, the fouling and corrosion potential are substantially decreased thus allowing the heat recovery system to be operated at a lower outlet flue gas temperature which yields an incremental improvement in energy recovery.

This is a Continuation of application Ser. No. 07/404,153, filed Sep. 7,1989, now abandoned, which is a Continuation-In-Part of application Ser.No. 07/224,419, filed Jul. 25, 1988, now U.S. Pat. No. 4,871,522.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to emission control equipmentfor fossil fuel fired power plants, and in particular, to a new anduseful method and apparatus for SO_(x), NO_(x), and particulate control.

2. Description of the Related Art

Current energy policy in the U.S. is based on expanded use of coal inutility and industrial applications. This must not compromiseenvironmental requirements however. Advanced control technologies areneeded to control the increase in pollutant emissions of coalcombustion. These pollutants include particulates, sulfur oxides(SO_(x)), and oxides of nitrogen (NO_(x)).

Fly ash and other particulate material can effectively be controlledusing baghouses or fabric filterhouses. U.S. Pat. No. 4,309,386 which isassigned to the assignee of the present invention, discloses a hotcatalytic baghouse which simultaneously removes particulate material andreduces the NO_(x) emissions. This patent describes coating the catalystonto the fabric of the baghouse filter media including the use of afabric filter in which the catalyst is woven into the fabric.

U.S. Pat. No. 4,793,981 also assigned to the assignee of the presentapplication describes the use of a hot catalytic baghouse whichsimultaneously collects sulfur oxides, nitrogen oxides and particulates.

Selective catalytic reduction (SCR) of NO_(x) by ammonia is awell-established art. Various catalyst types and shapes are utilized instand-alone reactors, whereby flue gas is mixed with small quantities ofammonia and then contacted with the catalyst. Also, sorbent or alkaliinjection into the boiler and the ducts is a common approach for sulfurdioxide (SO₂) control from flue gas. The location of the sorbentinjection may vary depending upon the type of sorbent.

U.S. Pat. No. 4,602,673 discloses an apparatus for preheating combustionair while simultaneously reducing NO_(x) contained in the flue gas. Bycombining a catalytic reactor with an air heater, a compact device ispossible according to that patent. However, the catalyst will have to bereplaced frequently because of erosion of the catalyst due to dustbuild-up. Another major problem not addresssed by this patent ispoisoning of the catalyst from SO_(x) in the flue gas. In addition, flyash erosion will reduce the effective life of a SCR catalyst.

A combination of a catalytic baghouse and a heat pipe air heater havingcatalyst coating on its heating surfaces is described in U.S. Pat. No.4,871,522, granted Oct. 3, 1989 which is assigned to the assignee of thepresent application and is incorporated herein by reference.

A need remains for an apparatus and method for simultaneous SO_(x),NO_(x) and particulate control which integrates the catalyst inside thefabric filters of the fabric filter house to protect the NO_(x)reduction catalyst from fly ash erosion and poisoning by sulfur oxidesor fly ash. There is also a need for an improved heat recovery system toallow for a lower outlet flue gas temperature to yield an incrementalimprovement in energy recovery. Further still a need remains for sorbentrecycling and reactivation to improve the overall sorbent utilizationwhile decreasing the feed sorbent requirements.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems by providing amethod and apparatus for controlling the emissions of three knownpollutants that are generated when burning fossil fuels such as coal.The three pollutants are sulfur oxides (SO_(x)), nitrogen oxides(NO_(x)), and particulates.

In accordance with the invention, a high-temperature fabric filter houseor baghouse is employed together with means for recovering heat from thewaste gases generated from the combustion of fossil fuels.Advantageously, the SCR catalyst is integrated inside at least onefabric filter of the filter house, but preferably in all the filters.The SCR catalyst may be located in the plenum of the baghouse, butinside the fabric filters is preferred. An ammoniacal compound in thepresence of the SCR catalyst selectively reduces NO_(x) to nitrogen andwater.

Since the catalyst location is downstream of the fabric filter, the fluegas is essentially free of particulates and the majority of the sulfuroxides so as to protect the NO_(x) catalyst. Moreover, by virtue of thecatalyst being in the high temperature fabric filter house, it is at atemperature which promotes optimum reactivity.

The high-temperature fabric filter house is positioned upstream of themeans for recovering heat. The dirty flue gas plus the sorbent andammonia are supplied to the high-temperature fabric filter house wherethese pollutants are removed.

The "clean" flue gases are then supplied to the means for recoveringheat which includes a conventional gas/gas heat exchanger or acombination thereof including but not limited to heat pipes, rotatingdiscs, and rotating basket or even a gas/water heat exchanger where heatis transferred to the combustion air, or any other desired location.

Accordingly, an aspect of the present invention is to provide anapparatus and method of integrating an SCR catalyst into the fabricfilter house downstream of the particulate and sulfur oxides collection.Consequently the flue gas is free of particulates and the majority ofthe sulfur oxides have been removed before the flue gas contacts thecatalyst, thus protecting the NO_(x) reduction catalyst from fly asherosion and poisoning by sulfur oxides or fly ash.

Another aspect of the present invention is to enhance the thermalefficiency of steam generating boilers by providing a means to protectthe heat recovering means from acid deposition and corrosion. Thisadvantage is achieved by quantitative removal of sulfuric acid and SO₃in the hot baghouse which in turn reduces the sulfuric acid dew point toa value approaching the water vapor dew point or typically below 150° F.in coal fired boilers.

The various features of novelty characterized in the present inventionare pointed out with particularity in the claims annexed to and forminga part of this disclosure. For a better understanding of the invention,the operating advantages gained by its use, reference is made to theaccompanying drawings and descriptive matter in which a preferredembodiment of the present invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic block diagram of an apparatus in accordance withthe present invention for controlling emission of a fossil fuel firedboiler;

FIG. 2 is a cross-section in part of a high-temperature fabric filterhouse in accordance with the present invention;

FIG. 3 is a plot of percent lime utilization versus CA(OH)₂ :SO₂stoichiometry;

FIG. 4 is a graph of percent SO₂ removal versus injection temperature;

FIG. 5 is a graph of percent lime utilization versus injectiontemperature;

FIG. 6 is an illustration of a configuration of a SCR catalystemployable in the present invention;

FIG. 7 is an alternate embodiment of a SCR catalyst;

FIG. 8 is another embodiment of a configuration of a SCR catalyst; and

FIG. 9 is still another embodiment of a SCR catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the invention embodied therein comprises anapparatus for controlling the emissions of a fossil fuel fired boiler10, and in particular a coal-fired boiler. Boiler 10 includes aneconomizer 12 which receives combustion flue gases therefrom. The fluegases are provided on a flue gas stream in an exhaust duct 14 connectedto a fabric filter house or baghouse 16. Ammoniacal compounds areinjected upstream of the high-temperature fabric filter house 16 at 18.

Ammoniacal compounds is a term meant to include compounds such as urea,ammonium sulfate, cyanuric acid, and organic amines as well as ammonia(NH₃). These compounds could be used as reducing agents in addition toammonia. However, ammonia is preferred for economic reasons. Somenon-ammoniacal compounds such as carbon monoxide or methane can be usedas well, but with some loss in effectiveness.

Sorbent is injected into the boiler, either upstream at 20 or downstreamat 22 of the economizer 12. Depending upon the sorbent chosen, thesorbent is injected into the boiler, either upstream or downstream ofthe economizer. The term sorbent includes alkali both calcium-based orsodium-based including but not limited to calcium oxide, calciumhydroxide, calcium carbonate, calcium bicarbonate, calciumlignosulfonate, calcium silicate hydrates, sodium carbonate, sodiumbicarbonate, soda ash, nahcolite, sodium aluminate, trona, sodiumsesquicarbonate, and metal oxides of aluminum, copper, iron, and zinc.

For example, a calcium-based sorbent such as a mixture of hydrated limeand rehydrated recycled material are injected into the convective passof a boiler upstream of the economizer 12 where the flue gas temperaturemay range from 600°-1400° F. (315°-760° C.). The optimum temperaturewill vary with each sorbent. Simultaneous dehydration and sulfation ofthe sorbent begins immediately upon injection and continues as the fluegases pass through the boiler 10, economizer 12, and duct 14 into thehigh-temperature fabric filter house 16.

The fabric filter house 16 contains suitable fabric filters, such aswoven ceramic fabric bags, for example, 3M Nextel™ ceramic-woven bags.These high-temperature fabric filters enable the operation of the filterhouse 16 in the temperature range of 500°-900° F. (260°-400° C.).

The SCR catalyst 24 of which the composition is known in this art isincorporated into the filter house 16. As stated earlier, the SCRcatalyst 24 may be located in the exhaust plenum of the baghouse 16, butpreferably the SCR catalyst 24 is located inside the fabric filter 26 asis best seen in FIG. 2. The particulates and sorbent collects on thefilter bags 26 to form filter cakes where additional reactions with thesulfur oxides occur. By the time the flue gas reaches the catalyst 24,the sulfur dioxide concentrations have been reduced by about 70-90%. TheSO₃ concentrations have been reduced to below detectable levels, and theparticulates have been reduced to trace levels (99.9+% efficiency).

To facilitate integration of the catalyst 24 into the fabric filter bag26 of the fabric filter house 16, a baghouse with a pulse-jet cleaningsystem is utilized. In a pulse-jet baghouse 16, particulate filtrationoccurs on the outside of the bag 26, thus permitting incorporation ofthe catalyst 24 inside the bag 26 where in the presence of the SCRcatalyst, such as a Zeolite catalyst for example, the ammonia (NH₃) willselectively reduce NO_(x) to nitrogen and water according to thefollowing equation:

    4NO+4NH.sub.3 +O.sub.2 →4N.sub.2 +6H.sub.2 O        (1)

In the above example, the catalyst is effective in the temperature rangeof 480°-970° F. (250°-520° C.). A catalyst temperature range of700°-800° F. (370°-425° C.) is preferred at an NH₃ :NO_(x) stoichiometryof 0.9.

In the baghouse 16, the particulate formed during the combustion processand by the chemical reaction between the SO_(x) and the sorbent arecollected at the surface of the bag filter 26. The pulse-jet cleaningcycle of the baghouse 16 cleans the surface of the filters 26 anddischarges the particulates and sorbent into hoppers 56. The particulateor ash from the baghouse 16 can be discharged at 28 to dry disposal orsupplied to the recycle unit 32 by line 30. Since the solids collectedin the baghouse 16 contain fly ash, sulfated sorbent, and unreactedsorbent, recycling of the sorbent provides additional opportunity forreaction with the sulfur oxides.

To obtain recycled reactive solids, the recycled material may requiremechanical attrition and/or rehydration performed in a known fashion.The sulfated product layer is removed by mechanical means such asgrinding, attrition, or pulverization. The rehydration is achieved atatmospheric or pressure conditions using a batch or continuous hydrationtechnique.

The reactivated recycled solids are then injected into the flue gas foradditional gas/solid contact time. These recycled solids may even behydrated with the fresh sorbent by line 34, or in a separate processstep. If hydrated separately, the sorbent particles in the recyclesolids can vary in chemical composition from the fresh sorbent. Thelocation for recycled solids injection can be in a different temperaturezone from the fresh sorbent.

The now clean flue gas proceeds along the duct 36 to the means forrecovering heat 38. Advantageously, the process of the present inventionimproves the heat recovery and boiler efficiency since SO₂, SO₃, and theparticulates have been removed upstream of the means for recoveringheat. The fouling and corrosion potential, i.e., acid dewpoint aresubstantially decreased. This depressed acid dewpoint allows operationof the heat recovery means 38 at a lower flue gas exit temperaturethereby increasing heat recovery. Thus, the heat recovery system of thepresent invention is operated at a lower outlet flue gas temperature,yielding an incremental improvement in energy recovery. In theconventional system without SO₂, SO₃, and particulate control, the fluegas temperature is maintained about 250°-300° F. in order to avoid theacid dewpoint. In the present invention, the exit gas temperature can besafely lowered to 175°-200° F. (80°-95° C.) which represents anincremental energy recovery of about 1-3% of the total boiler heat load.

In the present invention, an additional heat transfer device recoversthe additional available flue gas energy. In a conventional boilersystem, the amount of energy removed in the air preheater is controlledby the flow rate and temperature of the primary and secondary combustionair. To recover this incremental amount of energy made available throughthe present invention, an additional heat transfer device may also beused. The additional device includes a combination of a conventionalgas/gas heat exchanger such as a heat pipe, rotating disc, or rotatingbasket and a gas/water heat exchanger. With this improved heat recovery,the present invention increases rather than decreases the cycleefficiency of the boiler. The clean flue gas exits along duct 40 to thestack 42 where it passes into the environment.

The system of the present invention employs a combination of ahigh-temperature catalytic fabric filter house, an improved heatrecovery system, and a recycle system for reactivating the unusedsorbent to provide for lower capital costs for initial installation andoperating costs thereafter. Furthermore, the present invention findsapplicability to coal-fired units and reduces the problems due to SO₂poisoning of the catalyst, catalytic oxidation of SO₂ to SO₃, anderosion and fouling of the catalyst by fly ash. The present inventionprovides a solution to these problems by placing the sulfur oxide andparticulate removal systems upstream of the SCR catalyst whileadvantageously maintaining the temperature of the SCR catalyst 24 at itsoptimum reactivity.

Recently completed pilot-scale tests using a 3,000 ACFM high-temperaturefabric filter house evaluated calcium-based sorbents and the viabilityof a NO_(x) reduction catalyst in the system of the present invention.The results of these tests indicated that hydrated lime injection ateconomizer temperatures is feasible. The overall sulfur dioxide removalimproved with increasing injection temperatures. Yet, SO₂ removals fromabout 70-90% were achieved at a Ca(OH)₂ :SO₂ stoichiometry of 2. Testsfurther indicated that NO_(x) removals greater than 80% were achieved atan NH₃ :NO_(x) stoichiometry of about 0.9.

The acid dewpoint of the flue gas at the baghouse outlet was less than170° F., which was the lower detection limit of the acid dewpoint meter.Accordingly, the system of the present invention potentially operates atair preheater outlet temperatures much lower than conventional systemswithout sulfur dioxide control.

In these tests, high calcium, commercial-grade lime was employed as thesorbent. The three variables that most influenced SO₂ removal were theCa/SO₂ stoichiometry, injection temperature, and bag cleanliness/filtercake thickness.

FIG. 3 shows the influence of the Ca/SO₂ stoichiometry. Even thoughthese tests were conducted at an injection temperature below the optimumtemperature, FIG. 3 illustrates that the SO₂ removal performance can benormalized by stoichiometry over the range of stoichiometries tested (1to 4). Sorbent utilization is equivalent to the SO₂ removal efficiencydivided by the Ca/SO₂ stoichiometry.

FIG. 4 depicts the effect of temperature at the point of injection uponSO₂ removal ranging from 720°-980° F. (380°-520° C.). SO₂ removalincreased significantly as the temperature rose.

FIG. 5 shows the influence of bag cleanliness on lime utilization. TheSO₂ performance always diminished following bag cleaning, probably dueto less filter cake on the bags. The baghouse cleaning techniqueconsisted of cleaning one row of bags at a time with the entire cyclelasting about 3 minutes.

An explanation for the apparent synergism between SO₂ reaction andthermal decomposition is that upon injection of calcium hydroxide intothe flue gas both SO₂ reaction and dehydration commence immediately inthe following manner:

    Ca(OH).sub.2(s) +SO.sub.2(g) →CaSO.sub.3(s) +H.sub.2 O.sub.(g) (2)

    Ca(OH).sub.2(s) +heat→CaO.sub.(s) +H.sub.2 O.sub.(g) (3)

The decomposition product, CaO, may react further with SO₂ as follows:

    CaO.sub.(s) +SO.sub.2(g) →CaSO.sub.3(s)             (4)

In addition, the CaSO₃ reaction product may oxidize to CaSO₄ :

    CaSO.sub.3(s) + 1/2O.sub.2(g) →CaSO.sub.4(s)        (5)

FIGS. 6-9 are four configurations of the SCR catalyst designed to beinserted into each bag 26 of a high temperature fabric filter house 16shown in FIG. 2. These catalysts are used in a baghouse 16 employing"outside" filtration, such as, in a pulsed jet baghouse. FIG. 6 depictsa catalyst bed 44 formed by two concentric cylinders 46, 48 each beingconstructed of a porous material such as a perforated metal plate. Thewidth of the gap created between these two cylinders 46, 48 are at leastone inch, but probably less than 3 inches. The catalyst is placed in thespace between the cylinder 46, 48 preferably by pouring it into place.Any poured catalyst, such as Raschig Rings, may be utilized, butpreferably the catalysts are Zeolite.

An alternate embodiment of this configuration is a highly porous,monolithic, thick-walled cylinder catalyst which serves a dual purposeof catalyst and bag retainer.

FIG. 7 depicts a parallel plate-type catalyst configured to fit into aconventional bag retainer. The flue gas, upon entering the bag 26, flowsup through the wedge-shaped passages 50 coated with the SCR catalyst.Alternate embodiments of this configuration are either as an extrudedmonolith or metal plates coated with the catalyst.

FIG. 8 depicts a catalyst monolith 52 placed at the top of each bag 26and through which a bag blow-back tube 54 extends. The shape of thepassages through the monolith 52 is arbitrary, but honeycomb is thepreferable shape. Alternately, the monolith catalyst 24 may be placedabove the tube sheet 58 immediately over each bag exit.

FIG. 9 depicts a configuration in which each bag consists of a doubledwall retainer formed by two concentric cylinders, 46, 48 similar to theconfiguration shown in FIG. 6. However, in this embodiment, the catalystis placed inside the second cylinder 48. The gap between cylinders 46,48 provides an unobstructed passage for blowback gas during the cleaningcycle. A check valve arrangement (not shown) prevents flue gas fromentering the blow-back region.

While a specific embodiment of the present invention has been shown anddescribed in detail to illustrate the application and principles of theinvention, it will be understood that it is not intended that thepresent invention be limited thereto and that the invention may beembodied otherwise without departing from such principles.

We claim:
 1. An apparatus for controlling emissions of a fossil fuel fired boiler which produces flue gases containing SO_(X), NO_(X), and particulates, comprising:a flue gas duct constructed so as to carry flue gases from a boiler to a stack for discharge; a high-temperature pulse jet fabric filter house connected along the flue gas duct between the boiler and the stack, said filter house constructed so as to remove particulate from the flue gas passing along the flue gas duct, said fabric filter house having a plurality of fabric filter bags contained therein with each of said fabric filter bags having a bag retainer situated therein; a selective catalytic reduction catalyst positioned inside the bag retainer of each of said fabric filter bags in said filter house; means for recovering heat connected along the flue gas duct downstream of said fabric filter house, said heat recovering means constructed so as to be heated by the flue gases in the flue gas duct; means for injecting an ammoniacal compound into the flue gas duct upstream of said filter house; and means for injecting sorbent into the flue gas duct upstream of the filter house whereby the sorbent reacts with SO_(X) from the flue gas, the particulates are removed in said fabric filter house, thus protecting the selective catalytic reduction catalyst from fly ash erosion and SO_(X) poisoning.
 2. An apparatus according to claim 1, wherein said high-temperature pulse jet fabric filter house has an outlet for particulates, the apparatus further including means for recycling sorbent, said recycling means being connected to the particulate outlet of said fabric filter house, and constructed so as to receive particulates therefrom, and said recycling means being connected to the means for injecting sorbent, and constructed so as to supply recycled sorbent to the flue gas duct upstream of the filter house.
 3. An apparatus according to claim 1, wherein said catalyst is situated in an upper portion of said bag retainer in said fabric filter bag, said catalyst further having a plurality of passages defining a honeycomb shape.
 4. An apparatus according to claim 1, wherein said catalyst further comprises a catalyst bed situated between two concentric cylinders with each of said cylinders being constructed of a porous material. 